1. Field of Application
The present invention relates to a method of image display in which data representing a single main image are held in a main memory and in which a portion of the data are selectively transferred to a display memory, to be used in displaying a portion of the main image while enabling scrolling of the displayed portion across the main image.
The term "main image" is used herein in a very general sense, e.g. to include a single graphic image, a single page of text formed of characters, an array of multiple pages of text or graphics, etc.
2. Prior Art Technology
In the prior art, methods of image display are known in which data representing a main image are held stored in a main memory, and in which a selected portion of the data are transferred temporarily to a display memory, and the contents of the display memory are displayed by a display device. In general, the amount of image data which can be displayed at one time by the display device is substantially smaller than the main image data held in the main image memory, so that only a small portion of the contents of the main image memory are transferred to the display memory, i.e. the display memory will generally have a smaller storage capacity than the main image memory.
FIG. 1 is a conceptual diagram for illustrating the basic principles of such a prior art method of image display. Numeral 401 denotes a main image memory, having data representing a main image stored therein. 402 denotes a region within the main image memory 401 in which is stored a set of data representing a portion of the main image, with the data 402 having been transferred to a display memory 403 to be temporarily held therein, while being displayed by the display device 406. It will be assumed that the system includes hardware whereby it is possible to selectively display the data held in the display memory 403 in a non-expanded mode, in which those data fill the display area of the display device 406, or an expanded mode, in which a portion of the data held in the display memory 403 (indicated by the broken-line rectangle 405) are displayed such as to fill the display area. In that case, for example assuming a condition of 1:1 relationship between respective image data and the pixels of the display device 406 in the non-expanded display mode, expansion by a factor of 2 (in the horizontal and vertical display directions, referred to in the following as the X and Y directions respectively) would signify that each datum of the region 405 of the display memory 403 will correspond to a specific set of four pixels of the display device 406. Designating the aforementioned expansion factor as m, the expanded image that is displayed in that case by the display device 406 is defined by the value of m and the values of the initial display coordinates Dpx, Dpy, which define a specific datum currently held in the display memory 403 as a display starting point, with respect to the X (horizontal) and Y (vertical) display directions.
More specifically, each datum held in the display memory 403 is defined by a pair of X and Y address coordinates. When the region 405 is displayed in expanded form, the first (e.g. uppermost) line of data of a frame displayed by the display device 406 will be read out from addresses in the display memory 403 which have the Y-coordinate DPy, and succeeding display lines of the frame will be obtained from addresses having successively increasing values of Y-coordinate.
With such a prior art method of image display, it is necessary to transfer large amounts of data between the main image memory 401 and the display memory 403 each time that a change is to be made in the picture displayed by the display device 406, e.g. each time that scrolling movement of the displayed portion of the main image is executed. For example, if a change is to be made from displaying the data of region 402 of the main image memory to displaying the data of region 402', then it is necessary to transfer all of the data of the region 402' to the display memory 403.